High pitched “whistle” on channel 1 “pushed” with any gain?

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Exodus5

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I just bought a mid-2000’s Dual Rectifier (3-channel) and it sounded OK when I tried it out but after the long car ride home When I turned it on at home I found that any time I hit a chord (or note) on channel 1 while in pushed The output changed to a hug pitched whistle/squeal. I checked my connections, etc, but finally realized that the squeal “followed” the channel 1 gain control. When I’m playing clean there is no squeal, but is soon as I push it over to pushed I find any amount of gain above about two brings about the loud squeal. If I start out with no gain and start playing and slowly turn up again at about 1/4 the output turns into a squeal and that squeal only changes in pitch the more I turn up the gain. What the heck is happening here?
 
Update: Nevermind. Apparently V1 was junk. Replaced it and we are back to normal.
 
That particular quirk of channel 1 in the pushed mode actually makes it very handy for testing preamp tubes. I use it for exactly that purpose.

In general, any tube that whistles in channel 1 pushed, before gain is at 10, is probably an old, well used tube that will not provide satisfactory performance in any high gain application. I might set aside a few for use as phase inverters or reverb circuit tubes. They're going to be junk if used in a high gain stage.
 

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